Kill summer weeds before they germinate

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Normally during February or early March I remind you that it is time to apply a preemergence herbicide to your lawn to prevent summer weeds from germinating.

Germination of summer weeds can vary from year to year, but on the average, the best time to apply a pre-emergence herbicide to the lawn is the latter half of February through the first week of March. It is usually during this time when the soil temperature normally stabilizes above 55 degrees. Some summer weeds, such as crabgrass, begin germinating at 55 degrees.

During this time of the year, we can see a lot of fluctuations back and forth with the soil temperature. It goes above 55 only to fall back after a cold front. How can you keep up with the soil temperature? Go to georgiaweather.net and enter your zip code for the closest weather station to you. For most of us in the Augusta area, the one at Dearing will be more accurate than the one at Clarks Hill because the Clarks Hill station is on the river bank behind the dam making it a cooler spot. After you get on the site, click on “31 Day Summary.”

This year, though, you might want to do things a little different if you have not already applied your preemergent. Many of us in my business agree that due to the cold temperatures of anywhere from 9 to 12 degrees as a low that we had in January that it might be better for the health of your grass if you apply half the normal rate of preemergence herbicide. The reason is potential winter kill or injury. Not so much on zoysia, but on centipede, St. Augustine, and possibly even Bermudagrass (mainly on shorter cut Bermuda).

The weather from now until mid to late March will dictate the extent of winter injury.

Based on my many years’ experience, I believe the most winterkill we get on centipede and St. Augustine is not so much the absolute cold we get in the winter, but the late spring frosts we get in March after the grass begins greening up. It is therefore safe to conclude that the winter we had has set us up for a potential problem if we have a real cold spell in March, such as the low to mid 20’s.

Winter injury is seldom an all or nothing phenomenon. It can range from our grasses being a little slower than normal to green-up, to isolated dead spots, to complete kill. It will typically be worse in shady or wet areas and north facing slopes.

Anytime there is a heightened concern of winter injury, it is wise to reconsider the application of your preemergence herbicide. Because winter injury results in turf needing to be grown back and fill in bare areas, full rates of some herbicides can slow down the process. Applying these herbicides in February or early March is to do so before the extent of winter injury or before injury that might occur later in March.

The typical preemergence herbicides we buy at the garden centers are what we call dinitroaniline herbicides (pendimethalin, dithiopyr, benefin, etc.). They can inhibit root growth on stolons as turf recovers and grows into thin areas. Inhibiting stolon rooting may cause stolons to be cut off during mowing, significantly reducing lateral spread and recovery.

Researchers at Universities’ Research Stations have shown these herbicides can be safely used on thin warm season grasses if they are used at reduced rates (usually one half the normal rate). Therefore, anytime winter injury is a concern, it is important to split the applications of the herbicide. For example, if the directions say to apply 3 pounds per 1000 square feet, use 1.5 pounds. If your grass looks pretty good after green-up, apply the other 1.5 pounds about 60 days later. If it does not, skip the 2nd application.

Fertilizing too early can also weaken the grass. As I have preached for years, I have said to wait until about April 15 to fertilize your lawn as it’s important to wait until your grass is growing strongly and when root growth occurs. Typically that means waiting until the soil temperature stabilizes above 65 degrees at a 4 inch depth. On the average this occurs about April 15.

I know this goes against all the pre-Masters preparation, and sometimes the soil temperature is plenty warm by the 1st of April, but we never know what kind of cool spring we may have leading up to that big week in early April.

Reach Sid Mullis, the director of the University of Georgia Extension Service office for Richmond County, at (706) 821-2349 or smullis@uga.edu.


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